Seminars by M.Tech/M.S students

Date

Name

Course

Abstract

02.02.2018

Allaparthi Venkata Satya Vithin

M.Tech

Title: Femtosecond laser based microstructures fabrication for development of miniaturized Photonic devices

In the past few decades, significant effort has been put forth in miniaturization of different optical devices identified as Lab-on-chip systems. Recently, Optofluidics, the merger of optics and microfluidics, appears to be an ideal platform for realizing fully integrated optical sensors. The advent of femtosecond laser micromachining for fabrication on transparent material has opened up new avenues for development of optofluidic lab-on-chip systems. Femtosecond micromachining can also be used for direct fabrication of high speed optical modulator by using electro optic materials. Present seminar going to discuss basic of femtosecond laser micromachining in order to fabricate such lab on chip based devices including some primary results obtained in the lab.

02.02.2018

Ashitosh Venkatasai Velamuri

M.S

Title: Bend Loss Measurements in Optical Fiber

Optical fibers when installed in the form of cables are bent, in case of FTTH applications where they are bent at the tight corners of the walls and in the design of integrated optical system are the fibers are bent to provide geometric displacements.In this talk I would discuss about the effecting of bending on the transmitted power through optical fibers, simulation strategies applied and the results obtained, and then compare them with experiment. I am going discuss how better simulation techniques were applied when compared with the previous works. Here we use GEBT (Geometrically Exact Beam Theory) to obtain stress tensor of a single-mode fiber that is bent and twisted. The stress tensor is used to calculate modified refractive index of the fiber which is then used to numerically calculate bend losses in the fiber. Then I would like to give a brief introduction to micro bending losses, how do they come in to picture and then also how these losses effect the transmission.

Mode-Locked Lasers (MLLs) are used to generate ultrashort optical pulses and are used in a variety of applications such as high-speed optical communications, nonlinear spectroscopy, etc. The MLL is sensitive to thermal changes, mechanical vibrations, and environmental perturbations etc., which causes the laser to detune from its ideal locked condition. This is termed as detuning of the laser which increases pulse width of the laser pulses and distorts the pulses. The study of laser behavior when it is detuned is important to understand pulse stability and infer the locking range or detuning bandwidth of the laser. In this talk, I will discuss a novel technique to increase the detuning bandwidth of actively mode-locked fiber lasers, by subcarrier modulated RF signal to modulate the cavity loss. Employing Kuizenga-Siegman theory, we derive expressions for pulse width and pulse position and study the effect of detuning on these two parameters. We validate the theoretical results by experiments on actively mode-locked erbium-doped fiber ring laser with fundamental pulse repetition rate of 10 GHz and pulse width approximately 37 ps. We obtain a detuning bandwidth of 420 KHz without subcarrier modulation. Later, by employing our novel technique of cavity loss modulation, we report an increase in detuning bandwidth up to 1 MHz, an improvement by a factor greater than two. Our theory can potentially serve as design guidance for cavity length feedback control of harmonic ode-locked fiber lasers.

Mode-Locked Lasers (MLLs) are used to generate ultrashort optical pulses and are used in a variety of applications such as high-speed optical communications, nonlinear spectroscopy, etc. The MLL is sensitive to thermal changes, mechanical vibrations, and environmental perturbations etc., which causes the laser to detune from its ideal locked condition. This is termed as detuning of the laser which increases pulse width of the laser pulses and distorts the pulses. The study of laser behavior when it is detuned is important to understand pulse stability and infer the locking range or detuning bandwidth of the laser. In this talk, I will discuss a novel technique to increase the detuning bandwidth of actively mode-locked fiber lasers, by subcarrier modulated RF signal to modulate the cavity loss. Employing Kuizenga-Siegman theory, we derive expressions for pulse width and pulse position and study the effect of detuning on these two parameters. We validate the theoretical results by experiments on actively mode-locked erbium-doped fiber ring laser with fundamental pulse repetition rate of 10 GHz and pulse width approximately 37 ps. We obtain a detuning bandwidth of 420 KHz without subcarrier modulation. Later, by employing our novel technique of cavity loss modulation, we report an increase in detuning bandwidth up to 1 MHz, an improvement by a factor greater than two. Our theory can potentially serve as design guidance for cavity length feedback control of harmonic ode-locked fiber lasers.When a high-power optical signal is launched into a fiber, the linearity of the optical response is lost. One such nonlinear effect, which is due to the third-order electric susceptibility is called the optical Kerr effect. FWM is a type of optical Kerr effect, and occurs when light of two or more different wavelengths is launched into a fiber. Applications so far use the propagation in unidirectional mode, where only one of the propagation directions is utilized. The main emphasis of our investigation is on bidirectional mode of operation, where both directions are used simultaneously for independent FWM operations. We propose a bidirectional FWM scheme to increase the transmission capacity of optical fiber. Simulation in MATLAB shows improvement in overall efficiency with less usage of pump power as compared to unidirectional case. We performed certain unidirectional FWM experiments to visualize this effects in single mode fiber. In future we intend to experimentally check the bidirectional FWM effects and compare it with numerical simulations

In a typical optical trap, light pressure from a tightly focused laser beam is used to trap microscopic particles. This has found a large number of application in biology, physics and other fields. Typical single beam optical tweezers use CW lasers. Here we present the principles of optical tweezers using a high repetition rate Femtosecond laser beam. We use the principle that the high peak power of each laser pulse is enough to provide an instantaneous trapping potential while the high repetition rate ensures the sustained stable trapping from the successive pulses. Since high peak powers are involved, the nonlinear optical effect becomes much more significant for femtosecond optical trapping. Consequently, trapping of spherical beads is analyzed using the additional nonlinear effect for the femtosecond laser case and is compared with CW trapping. Numerical modelling is used to find the feasibility of femtosecond laser tweezers and calculating the induced forces on the nanometer size spherical particle. This model can be used to evaluate the stability of optical traps under a variety of different conditions like power, beam parameters, Wavelength, beam waist etc. for the nanometer sized particle. Presently, the simulations are being advanced to the Ray Optics Regime and the Lorenz Mie Regime so that the simulated results can be correlated to the experimental results.

In a high speed optical communication system coupling loss in between OEIC devices and optical fiber is a major concern, In this thesis work a horizontal taper rib waveguide is to be fabricated on a multi-quantum well embedded wafer for which mask layout has been designed, further impurity free induced vacancy disorder is to be created by depositing and annealing the thickness varied ZrO2 cap that will cause quantum well intermixing more at interface and lesser down the length of waveguide hence band gap of waveguide will linearly decrease and refractive index will increase along the length whose effect will be similar to vertical tapering of rib. In order to deposit thickness varied ZrO2 piezoelectric crystal based moving knife edge system has been designed and calibrated with programmable dc voltage supply. Hence this vertical tapering effect by ZrO2 cap and horizontal tapering of rib at the coupling interface of the waveguide will increase the matching of optical fiber mode size with waveguide mode.

Fibre lasers have gained popularity owing to their simplicity of construction, versatility of applications, and the possibility of wide tuning range. But they are essentially non-linear systems which show a variety of time-dependent characteristics (dynamics) that could further widen their role in applications. This work starts with the study of an erbium doped fibre amplifier to understand the amplification characteristics of the erbium doped fibre. Then an erbium doped fibre laser is constructed with a ring cavity for 1550 nm range, to evaluate the modal properties of its linewidth. The relaxation oscillation frequency of the laser is measured by a simple approach. The spectral content aids the nonlinear dynamical features of the laser system to evolve when the intra-cavity loss is modulated ever so slightly by square- or sinusoidal-modulation at kHz frequencies. In addition to the experimental work, the theoretical model will be presented and the bifurcation diagram will be analysed.